A large static magnetic field is used by MRI scanners to align the nuclear spins of atoms as part of the procedure for producing images within the body of a patient. This large static magnetic field is referred to as the B0 field.
During an MRI scan, Radio Frequency (RF) pulses generated by a transmitter coil cause perturbations to the local magnetic field, and RF signals emitted by the nuclear spins are detected by a receiver coil. These RF signals are used to construct the MRI images. These coils can also be referred to as antennas. Further, the transmitter and receiver coils can also be integrated into a single transceiver coil that performs both functions. It is understood that the use of the term transceiver coil also refers to systems where separate transmitter and receiver coils are used. The transmitted RF field is referred to as the B1 field.
MRI scanners are able to construct images of either slices or volumes. A slice is a thin volume that is only one voxel thick. A voxel is a small volume over which the MRI signal is averaged, and represents the resolution of the MRI image. The term volume will be understood to refer to both slices and volumes.
When an MRI examination is performed, the MRI images that are constructed are temporally dependent. If an examination is performed on a region of the body which is moving, such as on the heart or an organ in the vicinity of the diaphragm, time dependent images need to be taken and correlated to the heart beat and the breathing cycle. Standard techniques in the art exist compensating for this motion. The anatomy of certain organs and portion of the body can also be challenging to image properly. For example, the heart and the knee require highly trained and high skilled operators to produce clinical MRI images which are useful for diagnosis.
For example, during an MRI examination of the heart the location and the orientation of the slices produced relative to the anatomy of the heart is critical for the specific diagnosis. To perform a MRI examination of this type, an operator would perform a series of scans. First, the operator images multiple slices of the patient's body to determine the rough orientation of the patient's anatomy. Using these rough images, the operator would manually locate the diaphragm and the heart. At a minimum a stack of slices would be orientated by the operator. In practice, navigator beam, a shim volume or other elements need to be orientated, which makes this a time consuming operation. The stack of slices defines the orientation, field of view and the series of slices with all their parameters which are used in the next scan. The navigator beam is a thin volume that is imaged to correlate the position of the diaphragm and compensate for the patient's breathing. The shim volume is a volume from which information is used to compensate for local fluctuations in the B0 field.
After these volumes are determined, a second series of images resolved for the phase of the patient's breathing and heart are acquired. This new set of MRI images is then used by the operator to examine the anatomy of the heart in greater detail and manually define, depending on the suspected disorder, the slices or volumes which will be imaged for clinical diagnosis by a physician. Images which are intended for the purpose of determining a diagnosis are referred to as clinical MRI images.
For the heart, setting the position and orientation of the stack of slices used to acquire the clinical MRI images will greatly depend upon the indication of the patient. For example, the visualization needed to properly diagnose congenital heart disease can be quite different from ischemic heart disease. To be able to properly set up the MRI system to image the correct MRI images, the operator will need a large amount of training, experience, and an ability to visualize three dimensional (3D) structures from two dimensional (2D) projections or slices on a computer monitor. The difficulty with this is that the clinical images obtained by different operators will have differing levels of quality. In addition, reproducibility of a clinical MRI image, e.g., in a follow up procedures, is difficult to achieve when setting the position and orientation of the stack of slices is performed manually. When an operator produces clinical images of poor quality, they will need to be redone. This is of course expensive for hospitals, and raises the cost of having MRI examination done on a patient. US patent application 2005/0165294 teaches the use of a three-step procedure to correct for the positioning of patients during medical scans.